In this paper we argue that smallholder livelihoods in southern Mexico could suffer a disproportionate negative impact if transgenic technology is privileged as a response to climate change in substitution of an agroecological and evolutionary approach for the conservation and use of maize landraces. Currently, commercial maize seed is planted on less than one-fourth of Mexico's 8 million hectares. The rest is farmer-saved seed–mainly traditional landraces and creolized commercial cultivars. Landraces conserved in situ are the product of past and contemporary patterns of natural and farmer mediated evolution. Mexican maize landraces experience evolutionary forces – such as gene flow, hybridization, selection, mutation, and genetic drift – and multiple factors organize neutral and adaptive genetic diversity. Mexican farmers have subsisted for centuries with landraces, but it is uncertain how, and to what degree, farmers and landraces will respond to climate change. Farmer practices may adapt and landraces may evolve (or plastically respond) to changing conditions in ways that compensate for negative effects. Yet there may be limits to such positive responses. In this context, there are strong a priori grounds on which to defend the rights of farmers to save, sow, exchange, and improve their maize. In recent years, promoters of biotechnology have argued that transgenic crops are a useful tool to combat climate change. Some argue that transgenic technologies are required for countries like Mexico due to concerns about delayed technology development and free trade—despite wide social rejection. Rejection stems in part from fears about multinational corporations’ control over the agricultural sector, and from the fact that more than 85% of the 2 million households producing maize with farmer saved seed would not benefit from the technology. In fact, many fear that the genetic integrity of landraces could be compromised. In addition, research on the human and environmental effects of transgenic maize has not addressed the conditions of maize production in Mexico, so risks could materialize for these same smallholder households. For a crop in its center of origin with enormous diversity and great economic and cultural value, many ecologists intuitively affirm the precautionary principle, but this principle has been aggressively rejected by industry. An agroecological and evolutionary approach to addressing detrimental effects of climate change on smallholder agriculture is essential in this context.